1. Field of the Invention
The invention relates generally to water treatment and, more particularly, the invention relates to the use of zeolite crystalloids as coagulants in the clarification of water or to promote improved retention, drainage, and formation in the manufacture of paper and paperboard.
2. Description of Related Technology
Considerable expenditures are made each year for materials used in water treatment, such as coagulants and flocculants used to clarify raw waters for potable and industrial use; to clarify process water streams such as deinking water in paper recycling plants and mining wash water circuits; and to clarify industrial and municipal effluent waters, for example. Coagulants and flocculants are used to improve the efficiency of the papermaking process by improving the first pass retention of fiber and filler, and formation and drainage, on the paper machine forming wire. Therefore, the demand for more efficient, inexpensive water treatment compositions for use as coagulants and flocculants is increasing.
Widely used as coagulants and flocculants in the clarification of raw waters, process waters, and effluent waters are chemicals such as alum, sodium aluminate, polyaluminum chloride, activated silica, bentonite clays, inorganic iron salts, low molecular weight organic cationic polymers, high molecular weight organic polymers, and others. Some of these coagulants and flocculants may be used by themselves; however, in many cases various combinations of these materials may produce more efficient results, depending upon the nature and requirements of the water to be clarified.
The manufacture of nearly all fine paper worldwide is now done at alkaline pH levels (about 8.0 to 8.4 pH) using calcium carbonate pigment as a filler along with bleached wood cellulose fiber (some cotton cellulose is used in specialty papers). A current favored method of obtaining good sheet formation, good first pass retention of fiber and filler on the traveling formation wire, and rapid water drainage through the wire is to add rather high dosages (often three lbs per ton of dry furnish or more) of a synthetic high molecular weight cationic retention aid polymer flocculant to the machine furnish stream at the fan pump so as to utilize the high mixing forces at that point to deflocculate. Some believe so-called microflocs are, in part, created with this technique. However, if the use of the flocculant in this manner is all that is done, the retention of fiber and filler would be poor.
To provide superior retention, drainage, and formation, a coagulant is added before, or after the rotary screens located between the fan pump and the paper machine headbox. Generally known as microparticulates and of negative charge, these coagulants act as a bridge between the fibers, fines, and fillers.
The first such coagulant aid in wide use in papermaking was a bentonitic montmorillonite clay. Also now in use are colloidal silicas, modified colloidal silicas, polysilicate microgels, and very low molecular weight organic polymers, which are suggested to be sols on the basis of the very small water droplets emulsified in an oil carrier during the manufacturing process.
Regarding the use of microparticulates and cationic polymers as coagulation and retention aids in the manufacture of paper, U.S. Pat. No. 4,753,710 teaches that bentonite is essential. U.S. Pat. No. 4,753,710 compares bentonite to the aluminum silicate-modified surface on colloidal silica that is taught in WO 86/05826 and U.S. Pat. No. 4,980,025, and found bentonite to be superior. U.S. Pat. No. 4,753,710 teaches the use of at least four pounds bentonite per ton of furnish along with as much as three pounds cationic polymer flocculant per ton of furnish. Bentonitic montmorillonites are described as being metal silicates wherein the metal may be aluminum, and the ratio of silicon atoms to metal atoms is in the range of 5:1 to 1:1. This patent does not disclose that montmorillonites are layered minerals having tetrahedra on their top and bottom layers and an octahedral layer in the center. In the case of 1:1 (atomic ratio) silicon to aluminum, most of the aluminum is in the center octahedral layer where the cationic exchange sites are not available for cationic bonding. The overall surface area of a water swelled montmorillonite can be very large, reportedly as much as 800 m.sup.2 /gram, but most of the surface is disposed between layers where it as accessible to infused water but not available to participate in bonding to other particulates having calcium on their surfaces.
In another use of aluminum silicate as coagulant microparticulates, U.S. Pat. No. 4,954,220 describes the use of sodium aluminate as well as acids and certain other materials added to sodium silicate solutions to cause the formation of anionic, water soluble polysilicate microgels. The goal of this disclosure was to create colloidal silica analogs that would cost considerably less than colloidal silica. This disclosure relies on the anionic charge of the polysilicate microgel to perform well along with the use of cationic polymer flocculant, and requires the use of a sodium silicate with a SiO.sub.2 /Na.sub.2 O weight ratio of about 3.3, which is the maximum currently available, apparently in order to provide the maximum commercially available amount of silica per pound in order to produce a polysilicate microgel. While a reaction time of 5% to 95% of gel time is stated, it appears the disclosure is not concerned with overall unit size, although it is claimed that the resultant reaction product consists of three dimensional strings of 3 nm particles. This disclosure (Example 5) implicitly describes the use of an Al/Si weight ratio of 0.3 and claims results equal to colloidal silica. This disclosure does not recognize nor does it claim any advantage in utilizing the cation exchange ability of certain aluminum silicates, namely zeolites.
U.S. Pat. No. 4,980,025 and WO 86/05826 describe the use of aluminum silicates as coagulants in the papermaking process, with emphasis on "greatest improvements" obtained with mechanical or unbleached chemical pulps where "dissolved wood or trash substances" interfere with other programs. This disclosure uses an inorganic sol or aluminum-modified silicic acid sol. The inorganic colloid is taught as consisting of colloidal particles having at least one surface layer of aluminum silicate or aluminum modified silicic acid such that the surface groups of the particles contain silicon atoms and aluminum atoms in a ratio of from 9.5:0.5 (19:1) to 7.5:2.5 (3:1). It is further stated that the benefit of the aluminum atom is to form what is termed as an aluminate ion Al(OH).sub.4 -! that provides a fixed negative charge. No cation exchange ability of a properly formed zeolite, which is an aluminum silicate, is either recognized or claimed. Also, it is taught that the ratios of silicon to aluminum should be from 19:1 to 3:1.
Pummer, in Das Papier, 27, Volume 10, 1973, pages 417 to 422, describes the use of aluminum silicates in paper making but in this instance the aluminum silicate is a relatively large particulate used as a filler in paper at levels beyond 2.5%, based on the dry weight of paper.